Prior to the past several decades, scientists generally believed that it was impossible to restore hearing to the deaf. However, scientists have had increasing success in restoring normal hearing to the deaf through electrical stimulation of the auditory nerve. The initial attempts to restore hearing were not very successful, as patients were unable to understand speech. However, as scientists developed different techniques for delivering electrical stimuli to the auditory nerve, the auditory sensations elicited by electrical stimulation gradually came closer to sounding more like normal speech. The electrical stimulation is implemented through a prosthetic device, called cochlear implant, that is implanted in the inner ear to restore partial hearing to profoundly deaf people.
At present, very few cochlear implant patients are able to enjoy music. This is due, in part, to the fact that the cochlear fitting programs that process delivery of certain sound frequencies through a selected electrode or electrodes do not compensate for errors in pitch allocation.
Within the cochlea, there are two main cues that convey “pitch” (frequency) information to the patient. They are (1) the place or location of stimulation along the length of a cochlear duct and (2) the temporal structure of the stimulating waveform. In the cochlea, sound frequencies are mapped to a “place” in the cochlea, generally from low to high sound frequencies mapped from the apical to the basilar direction.
Mapping an electrode array in a cochlear duct to the correct audio frequencies is complicated by differences in an individual's anatomy. In addition, the final implanted position of the electrode array tends to be variable, which lends an arbitrariness to a mapping scheme between an electrode contact and perceived sound frequency. Thus, an optimal fitting map between an electrode contact and a sound frequency can only be roughly estimated at the outset for each individual. The initial estimate typically is inaccurate for that individual.
Another complicating factor is that the position of each electrode is not very precise, i.e., there are only a limited number of electrodes, e.g., numbering about 16 to 24 electrodes, spread along the length of the electrode array, inserted into one of the spiraling ducts of the cochlea. Hence, mapping to a “place” within the cochlea is not precise and is limited by the resolution of the discretely placed electrodes.